Optical low pass filter

ABSTRACT

An optical low-pass filter so formed that a rectangular wave phase grating including a plurality of sets of laminae for introducing phase retardation, in which laminae in each set being arranged in parallel with each other and the laminae of different sets being arranged to cross with an angle to each other is inserted in an optical system of the type having a color-encoding filter means disposed in the light path of the optical system for, such as, a single tube color television camera, and high frequency components of an objective image on the color-encoding filter means are attenuated less than a predetermined level, to the image focused on the color-encoding filter means, a degree of blur always larger than the value calculated from the grating period of the color-encoding filter means is given and another image focused on a position distant from said image as far as a predetermined distance brings about no interference to produce another color striped pattern with said color-encoding filter means, regardless of f-number of the objective lens and the diaphragm value so as to remove the influence effected by interference between the luminance and chrominance signal.

United States Patent [191 Nishino et al.

[ 1 Oct. 30, 1973 OPTICAL LOW PASS FILTER [75] Inventors: HisashiNishino; Teruo Hosokawa;

Ikuo Hioki, all of Osaka, Japan [73] Assignee: Minolta Comera K. K.,Osaka-fu,

Japan [22] Filed: July 21, 1971 [21] Appl. No.1 164,757

[30] Foreign Application Priority Data July 28, 1970 Japan 45/65418 [52]US. Cl 350/162 SF, 178/54 ST, 350/317 [51] Int. Cl. G02b 27/38 [58]Field of Search l78/5.4 ST; 350/162 SF [56] References Cited UNITEDSTATES PATENTS 3,502,799 3/1970 Watanabe 178/5 4 ST 2,733,291 1/1956Kell l78/5.4 ST 3,620,598 3/1970 Brandt 350/162 SF X 3,619,034 1/1970Yagi et al l78/5.4 ST X Primary Examiner-John K. Corbin Att0rneyStanleyWolder [57] ABSTRACT An optical low-pass filter so formed that arectangular wave phase grating including a plurality of sets of laminaefor introducing phase retardation, in which laminae in each set beingarranged in parallel with each other and the laminae of different setsbeing arranged to cross with an angle to each other is inserted in anoptical system of the type having a color-encoding filter means disposedin the light path of the optical system for, such as, a single tubecolor television camera, and high frequency components of an objectiveimage on the color-encoding filter means are attenuated less than apredetermined level, to the image focused on the color-encoding filtermeans, a degree of blur always larger than the value calculated from thegrating period of the color-encoding filter means is given and anotherimage focused on a position distant from said image as far as apredetermined distance brings about no interference to produce anothercolor striped pattern with said color-encoding filter means, regardlessof f-number of the objective lens and the diaphragm value so as toremove the influence effected by interference between the luminance andchrominance signal.

5 Claims, 11 Drawing Figures Patented Oct. 30, 1973 3 Sheets-Shoe t 1FIG.

FIG. 2

mvgsmozz mse m m same 1: A-K\L0 HooKgwg tKuRou mom I wm Patented Oct.30, 1973 3 Sheets-Sheet 5 OPTICAL LOW PASS FILTER BACKGROUND OF THEINVENTION The present invention relates to an optical system having acolor-encoding filter means, for example, a single tube color televisioncamera, and an optical lowpass filter in an objective lens system forattenuating beats appearing on the television picture, more particularly relating to an optical system in which the optical low-pass filteris so formed that there is provided a rectangular wave phase gratingincluding a plurality of sets of laminae for introducing phaseretardation and laminae in each set being arranged in parallel with eachother and the laminae of a different set being arranged to cross with anangle to each other.

In the prior art, for example, in television cameras of a single imagetube or two image tube type, it has been well known to make use of anoptical system provided with a color-encoding filter means disposed inthe light path of the optical system so as to take out three primarycolor signals.

In such an optical system, for television camera, it has been also wellknown that a beat of color striped pattern comes out on the pictureimage in the color television due to mutual interference between colorgratings on the color-encoding filter and a picture image focused on thecolor-encoding filter by the objective lens system as a shape pictureimage.

In order to attenuate such a beat it is required that high frequencycomponents of the objective image on the color-encoding filter areattenuated less than a predetermined level. In order to satisfy thisdemand, on the optical system there is provided an optical low-passfilter to a given degree of blur always larger than the value calculatedfrom the grating period of the colorencoding filter to the objectiveimage on the colorencoding filter, regardless of f-number and thediaphragm aperture value of the objective lens system.

As such an optical low-pass filter, it has been known that a polyhedronprism, nylon mesh, or the like has been used, however, every one of themrequires trouble in manufacturing and also incorporating into thephotographic lens, and accordingly it is not suitable to practical use.

Whereas, this applicant has invented a unique lowpass filter composed ofrectangular wave phase-grating, on which transparent base plate laminaein equal width for giving a phase retardation 5 are regularly disposedwith identical pitches in an identical direction, and filed as thepatent application Ser. No. 161,454 filed on July 12, 1971 and havingthe same assignee as the present application. Said optical low-passfilter has the advantage of that the manufacturing is easy because itcan be obtained through the conventional vacuum evaporation method andit is easy to incorporate it into the objective lens, while it has thedrawbacks in that when a degree of blur to the image of the principalobject to be focused on the color-encoding filter takes place beyond apredetermined level, color striped patterns come out through anotherinterference between color gratings on the color-encoding filter and animage to be focused on a position distant from the focal plane for saidprincipal object.

The present invention aims to remove the abovementioncd drawbacks andprovide an optical low-pass filter in which all of object images focusedand not focused on the color-encoding filter bring about by no meansinterference between said images and the colorencoding filter.

OBJECTS OF THE INVENTION One object of the present invention is toprovide an optical low-pass filter which is put to use in an opticalsystem having an objective lens and a color-encoding filter means, andthat prevents color striped patterns effected picture images from comingout.

Another object of the present invention is to provide an opticallow-pass filter which is put to use in optical system having anobjective lens and a color-encoding filter means, and that forming nocolor striped pattern effected by interference to all of object imagesof said objective lens focusing or not focusing on said grating shapedcolor filter plane.

Further another object of the present invention is to provide an opticallow-pass filter which attains the above-mentioned objects and is easy inmanufacturing and in inserting into an optical system.

The other objects of the present invention will be cleared in thedetailed description disclosed hereinafter.

SUMMARY OF THE INVENTION In order to attain the above-mentioned objects,the present invention relates to an optical system having acolor-encoding filter means for example, a single tube color televisioncamera and an optical low-pass filter in an objective lens system forattenuating beats appearing on the television picture, wherein theoptical lowpass filter is so formed that there is provided a rectangularwave phase grating including a plurality of sets of laminae forintroducing phase retardation and laminae in each set being arranged inparallel with each other and the laminae of a different set beingarranged to cross at an angle to each other, and high frequencycomponents of an objective image focused on the color-encoding filtermeans are attenuated less than a predetermined level, by means soarranged that there is given to the image focused on the color-encodingfilter means a degree of blur always larger than the value calculatedfrom the grating period of the color-encoding filter means, and anotherimage focused on a position distant from said image insofar as apredetermined distance brings about no interference to produce anothercolor striped pattern with color-encoding filter means.

Through a rectangular wave phase grating having one set of laminae forintroducing phase retardation an image of a point focused on apredetermined position by the objective lens system turns to Fraunhoferorder diffraction image on a position distant from the focusing pointhaving the spread in the direction as that of laminae disposition,namely, in the direction at right angles to the direction along tolaminae, instead of one point image. Depending upon the width of spreadof said diffraction image, gradation of said focus image comes out, andwhen said width of spread is near by about twice the period of thecolor-encoding filter interference between said focused image and saidcolorencoding filter can be prevented.

However, in the case of a defocused image positioned on a distantposition from the focused point, diffraction striped patterns of theFresnel order come out, and when period of striped diffraction patternson a defocus plane approximate to the period of the color-encodingfilter interference comes out between said both, and as the result, inan apparatus making use of such an optical system, for example, for asingle tube color television camera, color striped patterns come out onthe television picture.

However, in the present invention another set of laminae arranged inparallel with each other for introducing phase retardation, and thelaminae of the set being arranged to cross at an angle to that of thefirst set in said direction can attenuate striped diffraction patternsof Fresnel order of the defocused image and color striped patternseffected by the interference between the color-encoding filter and thestriped diffraction patterns.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. I is a perspective view ofa phase plate including a rectangularphase grating having a set of laminae.

FIG. 2 is a section view along the arranged direction of laminae on saidphase plate.

FIG. 3 is a perspective view showing various diagramatically diffractionimages formed at various points through a rectangular phase grating andan objective lens system.

FIG. 4 is a perspective view showing diagramatially the phase where thestriped diffraction patterns of an image shown in FIG. 3 and the gratingpatterns of the color-encoding filter bring about interferencetherebetween.

FIG. 5 is a perspective view of one embodiment of the optical low-passfilter in accordance with the present invention, in which two sets ofrectangular phase gratings are disposed so that both sets of laminaecross at an angle to each other.

FIG. 6 is a perspective view of another embodiment in accordance withthe present invention, having a phase plate including two sets ofrectangular phase gratings, in which laminae in each set are arranged inparallel with each other and the laminae of different sets are arrangedto cross with an angle to each other on a single base plate.

FIG. 7 is a perspective view showing diagramatically diffraction imageson the focal plane and the defocus plane in case of that the opticallow-pass filter in said embodiment is fitted in the objective lens.

FIG. 8 is a front view showing one example of colorencoding filter.

FIG. 9 is a perspective view showing the formation of the essentialportions of an optical system for a single tube color television camerain accordance with the present invention.

FIG. 10 is a perspective view of an optical low-pass filter of anotherembodiment in accordance with the present invention.

FIG. lll is a perspective view of an optical system in further anotherembodiment in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. I and FIG. 2 show anoptical low-pass filter composed of a rectangular phase grating 5, whichconstitutes a part of an embodiment in accordance with the presentinvention and has the phase disposition direction in a single direction,wherein on base plate 1 lami' nae 2 are arranged in parallel with eachother, and provided that the grating pitch of the laminae 2 is X and thewidth of said lamina 2 is a, the space between adjacent laminae 2 isX-a. And, provided the phase retardation effected by said laminae 2 is 8between light ray 3 through lamina 2 and light ray 4 only through baseplate I phase difference 5 comes out.

FIG. 3 shows states of various images formed at various points through arectangular phase grating 5 and an objective lens system 6 and therebyobject point P on the optical axis of said objective lens 6 formsvarious images.

When an optical system 6 is not provided with optical low-pass filter 5,object point P forms a point image on its conjugate point throughobjective lens system 6; however, when optical system 6 is provided withlowpass filter 5, the image of object point P passes through objectivelens system 6 and low-pass filter 5 passes through said conjugate pointand brings about a diffraction image of the Fraunhofer order composed ofa plurality of point images spread in a straight line in the directionalong which gratings of low-pass filter 5 are arranged, onto plane A atright angles ot the optical axis. Through pattern width H of thisdiffraction image a blur appears on the image. When said width isbroader than about twice a grating period of a color-encoding filter forforming in succession a striped pattern, for example, of blue, green,red, and white as shown in FIG. 8, interference between said image andthe colorencoding filter can be prevented.

The fact described above is in the case of that the image point is onthe focal plane A of objective lens system 6 relative to object point P,however, when this is on the plane B or C distant from the focal planeA, a diffraction striped pattern of the Fresnel order comes out as shownin FIG. 3. And, a very clear diffraction striped pattern 7 comes out inthe striped pattern in the same direction as phase grating 2 and in thespread in the direction at right angles to said phase grating 2, on theplane D separated as far as specific length d from focal plane A asshown in FIG. 7.

When the period of said diffraction striped pattern 7 approximates tothe grating period of the colorencoding 8 in FIG. 8, interference comesout between them, and if such an optical system is put to use in a colortelevision camera, a beat in a color pattern comes out in regard to animage focused distant from the specific focusing point on the televisionpicture.

As to said phenomenon, according to our experiment, on a defocus imagein the case of that objective lens 6 is let out to a certain extent fromthe position where the focal point coincides, the first interferenceappears to the image, and when the objective lens system is movedfurther forwardly from that position where the first interferenceappears, the second interference is found.

Thereupon, in the present invention, as shown in FIG. 5 two rectangularwave phase gratings 5 and 5 each having base plate 1 or l and a set oflaminae 2 or 2' arranged in parallel with each other on base plate 1 or11' as shown in FIG. 11 and FIG. 2 are stacked upon' each other so thateach base plate l and ll is in parallel with each other and each laminae2 and 2 on different base plates 1 and l is arranged to cross at anangle to each other to form one optical low-pass filter, and providingthem so that phase elements 2, 2 may have an inclination to each other.

In such a low-pass filter, having two sets of laminae for introducingphase retardation, diffraction striped pattern 7 of the Fresnel order asshown in FIG. 3 and FIG. 4, which appear in the position D distant fromthe position A to be focused by phase grating 5 brings about aninterference with the diffraction striped pattern 7 of the Fresnel orderappears by phase grating 5 crossing to phase grating 5, so that cleardiffraction striped patterns of the Fresnel order shown on the plane Din said drawing are brought about by no means.

FIG. 6 shows another embodiment of the optical lowpass filter inaccordance with the present invention, which low-pass filter 9 includingone base plate 9 and striped pattern 9 having two sets of laminae asrectangular phase grating laminated on base plate 9 the laminae in eachset being arranged in parallel with each other and the laminae of adifferent set being arranged to cross at an angle to each other.

FIG. 7 shows the appearances of diffraction images on focusing plane Aand planes B, C, D certain distances distant from focusing plan A ofpoint image P on the optical axis through an optical system includingobjective lens system 6 and the optical low-pass filter 9 shown in FIG.6, and diffraction striped patterns generated on plane D in FIG. 7 whichcorresponds to plane D in FIG. 3 bring about no interference betweenthem and color encoding filter 8 shown in FIG. 4 and FIG. 8.

Therefore, in a single tube color television cameras and the like makinguse of an image forming optical system having color-encoding 8 providedwith said optical low-pass filter, the generation of color stripedpatterns on the television picture which is effected by interferencebetween object images which focused on the plane to be focused andcolor-encoding filter 8 can be removed as a matter of course, and alsothe generation of color striped patterns caused by diffraction stripedpatterns generated secondarily by object images which focused on a planedistant from the plane to be focused and the color-encoding filter 8 canbe removed as well.

FIG. 9 shows an applied example of the optical system including anoptical low-pass filter in accordance with the present invention in anoptical system for a single tube television camera, wherein an objectimage is formed on color-encoding filter 8 shown in FIG. 8 by objectivelens system 6 through optical low-pass filter 9, and said object imageis separated into picture elements of three primary colors blue, green,and red and said picture image separated is formed on the plane of imagetube 13 by field lens 11 and relay lens 12, and by scanning this thecolor signal of the picture image is taken out. In this case the opticallow-pass filter 9 is only required to be disposed not only in theposition of the pupil of objective lens system 6 but also in the frontor rear of said objective lens system 6, and also can be disposed asshown in FIG. 11.

In the description of the embodiment and the applied example disclosedhereinbefore, the color-filter is in making use of three primary coloredstrips in parallel with each other in one direction and as the result,now that the optical low-pass filter is in making use of two rectangularwave phase gratings 5 and 5 each having a set of laminae arranged inparallel with each other on each base plate, and each base plate beingin parallel with each other and each laminae being arranged to cross atan angle to each other as shown in FIG. 5, or a phase striped patternhaving two sets of laminae, the laminae in each set being arranged inparallel with each other and the laminae of a different set beingarranged to cross with an angle to each other as shown in FIG. 6, anybeat effected by interference between a focused image or a defocusedimage on the color-encoding filter and color stripes of thecolor-encoding filter can be perfectly removed, however, in the case ofa colorencoding filter having two sets of color striped patterns beingarranged to cross each other, it is required that the optical low-passfilter 14 including one base plate 14 and a striped pattern 14 havingthree sets of laminae for introducing phase retardation laminated onbase plate 14 the laminae in each set being arranged in parallel witheach other and the laminae of a different set being arranged to crossone another as shown in FIG. 10 is put to use, or the optical low-passfilter 5 formed with two rectangular wave phase gratings, one of whichhaving a set of laminae on the base plate as shown in FIG. 1, and theother of which having two sets of laminae on the base plate as shown inFIG. 6 are stacked upon each other so that the set of laminae of theformer is arranged to cross both sets of laminae of the latter.

Thus, by arranging to cross one set of laminae for introducing phaseretardation with the other set of laminae each time when sets of colorstriped patterns cross one another in the color-encoding filter isincreased by one, interference between diffraction image patterns of theFraunhofer order and the Fresnel order and colorencoding filter can beeliminated.

What is claimed is:

1. In an optical system for introducing an image of an object scene toan image tube in a color television system and having a color encodingfilter means disposed in the lightpath of said optical system andserving to spacially modulate at least two of three primary colorimages,

the optical system comprising optical low pass filter means disposedbetween said object scene and said color encoding filter and including aplurality of sets of laminae for introducing phase retardation, laminaein each set being arranged in parallel with each other and disposed atan angle to the laminae of at least one other set, and laminae in one ofthe sets being so arranged to attenuate beats effected by interrelationbetween said color encoding filter and high frequency components in saidobject scene.

2. An optical system as set forth in claim ll, wherein said low-passfilter further comprises a transparent base plate on which saidplurality of sets of laminae are mounted in combination with each other.

3. An optical system as set forth in claim 1, wherein each set oflaminae is respectively mounted on a transparent base plate to form alow-pass filter.

4. An optical system as set forth in claim 3, further comprising anobjective lens on both sides of which the low-pass filters are disposedperpendicularly to the optical axis of the objective lens.

5. An optical system as set forth in claim 1, wherein each lamina isrectangular in cross section, and each set of laminae is mounted on atransparent base plate in the form of a rectangular wave phasediffraction grating.

1. In an optical system for introducing an image of an object scene toan image tube in a color television system and having a color encodingfilter means disposed in the lightpath of said optical system andserving to spacially modulate at least two of three primary colorimages, the optical system comprising optical low pass filter meansdisposed between said object scene and said color encoding filter andincluding a plurality of sets of laminae for introducing phaseretardation, laminae in each set being arranged in parallel with eachother and disposed at an angle to the laminae of at least one other set,and laminae in one of the sets being so arranged to attenuate beatseffected by interrelation between said color encoding filter and highfrequency components in said object scene.
 2. An optical system as setforth in claim 1, wherein said low-pass filter further comprises atransparent base plate on which said plurality of sets of laminae aremounted in combination with each other.
 3. An optical system as setforth in claim 1, wherein each set of laminae is respectively mounted ona transparent base plate to form a low-pass filter.
 4. An optical systemas set forth in claim 3, further comprising an objective lens on bothsides of which the low-pass filters are disposed perpendicularly to theoptical axis of the objective lens.
 5. An optical system as set forth inclaim 1, wherein each lamina is rectangular in cross section, and eachset of laminae is mounted on a transparent base plate in the form of arectangular wave phase diffraction grating.